Diana Athill's flat, at the top of a house overlooking the less-visited slopes of Primrose Hill, is a snug eyrie of books and paintings, dark wood furniture and her own deeply coloured embroidery. The only sound is an electric fire, ticking and popping against the chill. "If you're an old woman, it's amazing what a little bit of makeup does," she says, returning with two cups of tea. "You put it on, and suddenly feel all put together." It's a point she makes early in Somewhere Towards the End, her new book about the losses, indignities and compensations of old age: how important appearance still is, if only to oneself; not to risk, say, scarlet lipstick, which will run into mouth wrinkles and make a woman look, as she observes of an old friend with not-quite-cruel accuracy, like a "vampire bat disturbed mid-dinner".

Days short of her 90th birthday - the phone keeps ringing, with friends organising lunches to celebrate - she could not seem further from any kind of blurring. There is something absolutely present about her. Her voice is poised between precise pre-war tones and grammar and an unforced modernity - a tension that exists in her work, too. She began writing at 43, with short stories, before discovering that she was a memoirist, and that even dressing up memoir as fiction was not for her. Her work is always described as honest (generally preceded by words like "painful", "terrible", "deplorable", "breathtaking"). Pain, fear and shame, happiness and unhappiness are faced up to and anatomised- not dismissed, but not allowed to rule, either. She calls it getting things right.

She is helped by a vivid memory. Her parents were not particularly happy (her mother had an affair and her father, a colonel in the British Army, was often away), but Yesterday Morning, Athill's account of growing up on her grandmother's Georgian estate in Norfolk, written when she was 85, is full of happiness. She and her siblings and cousins had the run of the grounds, eating moorhen's eggs, feeding pigs, damming streams. "Everything important in my life seemed to be a property of that place: the house and the gardens, the fields, woods and waters belonging to it. Beauty belonged to it, and the underlying fierceness which must be accepted with beauty . . . safety belonged to it, and so did my knowledge of good and evil, and my wobbly preference for good." Indoors, every teatime, their grandmother read to them: Ivanhoe, The Jungle Book, Jackanapes

This was the calm before the second world war, when, if you were young and didn't look too closely, the superiority of being upper class seemed as though it might last forever. Though Athill eventually rejected the "wicked nonsense" of these assumptions, she knows it was also an invaluable source of self-confidence. And she fell in love, at 15, with Tony Irvin, her brother's tutor; they went dancing ("mine was the first generation of country-house girls allowed to go to dances unchaperoned"), went sailing on the Broads, and became engaged when she was 19. The arrival of war changed everything. Irvin (called Paul in her expiatory memoir Instead of a Letter) was a bomber pilot stationed in Egypt, where she was to join him; when war was declared he was posted to Transjordan, and silence. Two years later, he wrote asking to be released from the engagement because he was going to marry someone else; not long afterwards, he was killed. It is the central wound in her life, and it took nearly 20 years to heal.

"My soul shrank to the size of a pea," she later wrote. "It had never been very large or succulent . . . but now it had almost shrivelled away. I became artful at avoiding pain . . . because what else could one do when one had understood that, as far as one's personal life was concerned, one was a failure, doomed to be alone because one did not merit anything else?" She slept as much as possible, slipped into meaningless affairs and one-night stands, often with married men who could not demand too much of her; she worked as a clerk in the Admiralty, then as a researcher at the BBC.

The most important thing was always her value as a woman, her erotic life, and that now was punctured and debased; a career was a distasteful adjunct, to earn a living - so it was a profound bit of luck that led her to Andre Deutsch at a party. Their affair was brief, but for nearly 50 years they were friends and colleagues at the two publishing houses she helped him found, and she was to become, accidentally, among the most acclaimed editors of the 20th century.

They published - then lost - Philip Roth (he found their advance for When She Was Good too small), and were served an injunction against publishing Norman Mailer's The Naked and the Dead. They were the first to publish VS Naipaul, Mordecai Richler, Brian Moore; Athill edited John Updike, Stevie Smith, Margaret Atwood. She refuses to be unduly nostalgic about any so-called golden age of publishing, though she accepts she coincided with an unusual window when "we were all thrilled at getting our hands on people from different places. We had a sort of feeling of an extraordinary world - at that time it was easier to get your book published if you came in from Nigeria or somewhere than if you were a young English writer. There was a sort of romantic thing about it."

Generally briskly unsentimental, she nevertheless had a kind of maternal weakness for the lost, the helpless, the unstable: she published and befriended "Didi", an Egyptian writer (and gambler, womaniser and depressive), for example, who moved into her home for five years and killed himself; and Hakim Jamal, a disciple of Malcom X who was murdered (this latter subject was the indirect reason for Naipaul's departure from Andre Deutsch: she felt his novel about it, Guerrillas, did not ring true, and told him so).

However, lack of confidence meant that for years she didn't attempt to write herself. "I always thought how lovely it would be to write, because of course I was living among writers. You see I'd always thought they were writers. I was someone who was sort of a handmaiden to writers." When writing came - one day in 1958, when she was 41, as a short story, fully formed, and then another and another - it was like a visitation. "And then I thought, I wonder whether anyone would like to read them? I didn't ever, at any point, sit down and think 'I'm now going to write something to be read'." Then one of them won an Observer short story competition. Which "gave me confidence, of course."

She had, however, been serving a kind of unconscious apprenticeship, through reading (the Cambridge critic Sir Arthur Quiller-Couch, for instance, and his championing of anglo-saxon words over latinate), and through editing - from Naipaul, for example, she learned how to be funny. But above all, she learned from Jean Rhys, whom Athill helped to rediscover in 1957, when Rhys had not yet finished Wide Sargasso Sea - a book which would very likely not have been published without Athill's perseverance and nursing, astute advice and friendship. Rhys would say, "'cut, cut, cut. Keep it down as much possible.' She used to overdo it, even."

Brevity, accuracy, lucidity - "the writing shouldn't come between the reader and what's being described. It should be as transparent as possible" - these virtues Athill applied to "Instead of a Letter", which she wrote in an intense rush when she was 43. In writing it she found her voice. In attempting to "get to the bottom of things, so I understood them better", in thinking bravely and unself-pityingly about premarital sex, and abortion, and depression, she inadvertently pioneered a kind of confessional memoir that would not, properly, take hold for another 30 years, and even then often would not match her.

In his introduction to Make Believe, Patrick French argues for Instead of a Letter as "an eminently realistic feminist text, which has aged better than its contemporaries such as The Golden Notebook and The L-Shaped Room." Certainly, she has little time for feminists such as Marilyn French, whom she also edited. "Marilyn French and her agent, two militant - and very charming - women. How they drove me mad."

"I think, on the whole, alas, it isn't just conditioning - I think that actually the difference between men and women basically is visible. Women actually have children and men don't. You cannot get away from that. So that when it comes to a sexual relationship, it has to be something different for a woman. Every time she fucks she could be completely changing her life. . . . Which is unfair - but it's life. And I think the good old die-hard women's lib sort of person did refuse to admit that."

Instead of a Letter was like late spring rain: it banished winter, and failure, and she bloomed. The book ends on a note of cautious happiness: she had met Barry Reckord, a Jamaican playwright. Eventually he left his wife (though not because of her, Athill insists); after eight years with Athill he had an affair with a young actress in her 20s, but by this time his relationship with Athill was no longer sexual, and the older woman invited the younger one to live with them. For two years, until the actress moved on to marriage and motherhood with another man, they lived a ménage a trois which Athill describes as among the happiest periods of her life.

The therapeutic instinct led her to write two more books, After a Funeral (1986) about Didi's death, and Make Believe (1993), about the Hakim Jamal incident, and that, she thought, was that - until Ian Jack, then editor of Granta, suggested she write about her time in publishing. Stet appeared in 2000, eight years after she had retired, and was another late blooming, giving her a measure of fame and acclaim. Yesterday Morning followed, and now there is her book about age, which is suffused by a striking contentment, even though it describes Reckord sinking further and further into illness, and her spending much time nursing. This has changed since the book went to press. He had a heart-attack, "and the heart-hospital said no, they wouldn't operate, because he would die. And I was really getting desperate. Because I am too old to cope with nursing."

She was unaware that a Jamaican niece and brother had been discussing what to do, and one day they descended and he went home with them. "I hope that he's happy - I'm afraid he's not. I've written and talked on the telephone and told him over and over again about how I really can't cope. He just says, 'When can I come back?' It's awful, really."

And yet, honesty compels her to say, the relief at a burden lifted outweighs the sadness, and satisfaction with her late success staves off fear and rage. Though "I do get worried from time to time, about how I'm going to deal with it when I can't manage any more. When I can't drive. I think what I probably might have to do, eventually is get myself into a sheltered home. I've got a wonderfully kind neighbour across the road, and I've got these darling nephews, but you can't depend entirely on others. And maybe when I reach that stage I'll get cross about life." She laughs, comfortably. "But at the moment, it's all right."